Abstract
FO-DTS (Fiber Optic Distributed Temperature Sensing) technology has been widely developed to quantify exchanges between groundwater and surface water during the last decade. In this study, we propose, for the first time, to combine long-term passive-DTS measurements and active-DTS measurements in order to highlight their respective potential to locate and quantify groundwater discharge into streams. On the one hand, passive-DTS measurements consist in monitoring natural temperature fluctuations to detect and localize groundwater inflows and characterize the temporal pattern of exchanges. Although easy to set up, the quantification of fluxes with this approach often remains difficult since it relies on energy balance models or on the coupling of distributed temperature measurements with additional punctual measurements. On the other hand, active-DTS methods, recently developed in hydrogeology, consist in continuously monitoring temperature changes induced by a heat source along a FO cable. Recent developments showed that this approach, although more complex to set up than passive-DTS measurements, can address the challenge of quantifying groundwater fluxes and their spatial distribution. Yet it has almost never been conducted in streambed sediments. In this study, both methods are combined by deploying FO cables in the streambed sediments of a first- and second-order stream within a small agricultural watershed. A numerical model is used to interpret passive-DTS measurements and highlight the temporal and spatial dynamic of groundwater discharge over the annual hydrological cycle. We underline the difficulties and the limitations of deploying a single FO cable to investigate groundwater discharge and show the impact of uncertainty on sediments thermal properties on the quantification of groundwater inflows. On the opposite, the active-DTS experiment allows estimating the spatial distribution of both the thermal conductivity and the groundwater flux at high resolution with very low uncertainties all along the heated section of FO cable. Our results highlight the added values of conducting active-DTS experiments, eventually combined with passive-DTS measurements, to fully investigate and characterize patterns of groundwater-stream water exchanges at the stream scale. The combination of both methods allows discussing the impact of topography and hydraulic conductivity variations on the variability of groundwater inflows in headwater catchments.
Highlights
Understanding groundwater and stream water interactions as integral components of a stream catchment continuum is crucial for efficient development and management of water resources (Bencala, 1993; Brunke and Gonser, 1997; Sophocleous, 2002)
240 3 Results 3.1 Passive-DTS measurements 3.1.1 Spatial variability of temperature signals Figure 4a synthesizes the results of the passive-DTS experiment and shows temperature signals monitored all along the FO cable deployed in the streambed sediments
Results highlighted preferential discharge areas depending on the streambed properties and topography, showing that the head of the watershed highly contributes to stream flow
Summary
Understanding groundwater and stream water interactions as integral components of a stream catchment continuum is crucial for efficient development and management of water resources (Bencala, 1993; Brunke and Gonser, 1997; Sophocleous, 2002). Essential for the preservation of groundwater dependent ecosystems and riparian habitats (Kalbus et al, 2006), these interactions play a major role on physical, geochemical and biological processes occurring in the stream or in the hyporheic zone (Frei et al, 2019; Jones and Mulholland, 2000). Localizing and quantifying exchanges between groundwater and stream water is often difficult as these exchange are controlled by multi-scale processes and are highly variable in time and in space (Brunke and Gonser, 1997; Fleckenstein et al, 2006; Flipo et al, 2014; Harvey and Bencala, 1993; Kalbus et al, 2009; Varli and Yilmaz, 2018; Woessner, 2000). The approach relies on the detection of temperature anomalies observed at the sediment-water interface (Tyler et al, 2009; Sebok et al, 2013; Westhoff et al, 2011) or into the streambed (Krause et al, 2012; Lowry et al, 2007)
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